This is caused by a hostname change.
When a page is created it holds an absolute reference to its page layout:
<mso:PublishingPageLayout msdt:dt="string">http://dummylink.stackemedia.co.uk/_catalogs/masterpage/aPage_Layout.aspx, A Page Layout</mso:PublishingPageLayout>
This needs to be updated to reflect the hostname change. To do this:
- Open the site in SharePoint Designer
- Check out the page
- Export the page – File > Export
- Open the content of the page and change the hostname as appropriate
- Return to SharePoint Designer and import the page – File > Import – overwriting the existing page
- Check-in the page and approve
